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As you can see, both 2x2 and 2x4 System bricks connect flush to Duplo bricks. This is because the tube on the underside of the brick fits neatly and snugly inside the hollow stud of the Duplo brick. You can attach a 2x3 System brick as long as it doesn't have the tube supports, but you'll have to stick it off one edge because it isn't long enough to cover both of the Duplo studs.
